The present invention relates in general to dental and jewelry casting equipment, and in particular to a new and useful positioner designed to position oval or other non-cylindrical investment molds into a casting machine designed to receive cylindrical investment molds only.
A wide variety of casting machines used in the jewelry and dental industry, include cradles for receiving cylindrical investment molds. Investment molds are formed of refractory material containing a mold recess shaped by a lost wax process, for casting metal jewelry and dental parts.
In about 1971, the assignee of the present application introduced oval-shaped casting rings which were used to form oval investment molds. These molds have superior cooling properties while using approximately 40% less investment material than conventional cylindrical molds.
Since dental bridges are generally straight in the posterior (back) of the mouth, dental restorations casted in round rings transverse various zones of expansion. Round rings expand more in the center than they do toward the sides of the ring. Round rings heat at a slower rate in the center than on the outer edges and they cool at a slower rate. The dynamics and advantages of using non-cylindrical, and in particular, oval rings is fully disclosed in an article authored by the co-inventors, "The Effects of Shape and Size on Investment Heating and Cooling Rates", Berger and Benson, J. of the Nat. Assoc. of Dental Laboratories, Volume 8, No. 3.
The dental industry has recognized the advantages of the oval-shaped rings but has had difficulty in the past adapting the unusually shaped molds to conventional casting machines. All casting machines manufactured world-wide are designed for various sizes of cylindrical rings. These rings vary from 2.75 to 3.25 inches in diameter. The various casting machines use supportive cradles for positioning these large cylindrical or round rings. Since the oval shape is so different from the round, a supporting ring positioner for the oval-shaped mold is either custom fit for each casting machine or else the oval mold is simply placed in the casting machine cradle in an unsecure position. This often results in unsuccessful casting operations. Examples of commonly used casting machines are the Kerr Centrifico Casting Machine, manufactured by the Kerr Company, Division of Syboron of Romulus, Michigan and the MODULAR 3 Casting Machine manufactured by Nobilium Company of Albany, N.Y.
An object of the present invention is to avoid the problems of securing a non-cylindrical mold in a casting machine designed for cylindrical molds only.